The invention relates of an apparatus for grinding especially for web thinning and split pointing of the tip of a twist drill.
Web thinning and split pointing of a sharpened tip of a twist drill bit is known in order to reduce the size of the transverse cutting edge to diminish axial forces, i.e., the forces necessary to advance the drill bit. In addition, web thinning and split pointing improves the cutting conditions at the transition from the transverse cutting edge to the main cutting edge. Therefore, web thinning and split pointing, i.e. the grinding of the core of the tip of the drill, has a substantial effect upon the cutting characteristics of a twist drill bit. It is possible to perform the web thinning operation by hand, although great dexterity is required to prevent errors. For that reason, twist drill web thinning and split pointing machines and devices have been designed to make possible the production of reproducible and precisely defined tip patterns on the drill bit.
The known helical drill tip web thinning and split pointing machines include a profiled grinding disk and a drill holding device that permits positioning the twist drill to be sharpened in a pre-determined spatial angular relationship relative to the grinding disk. To perform the grinding operation, the drill bit is oriented relative the grinding disk in such a way that the grinding disk profile, when viewed in the direction which is coincident with the intended direction of relative motion between the grinding disk and the drill holder, will produce the desired tip surface on the twist drill bit. The tip pattern will then be ground into the tip of the drill bit by reciprocal motion of the grinding disk or of the drill holder.
Due to the severe effect upon the cutting properties of the drill, the tip surface must be optimally adapted to the prevailing cutting edge geometry and the profile of the grinding disk which grinds the tip surface must be adapted to the prevailing cutting geometry. Accordingly, it is necessary, for example, to provide different profiled grinding disks for drills of different diameter, which makes the known tip grinding, i.e. web thinning and split pointing method expensive due to the relatively high expense for tools. Moreover, the required exchange of the profiled grinding disks causes tool exchange delays which add further costs to the known tip grinding method. In addition, the above-mentioned necessary high quality of the tip surface necessitates repeated dressing of the profiled grinding disks, thereby sharply increasing the amount of labor of the known tip grinding method and further increasing the cost of the overall method. Still further, the known tip grinding method involves high expense for machinery because expensive dressing machines are necessary to dress the profile disks to insure uniform grinding quality.